We designed and optimized gapless microlens for backside illuminated (BSI) CMOS image sensors (CIS) to improve optical properties. Gapless microlens can provide better light focusing on photo-diode than conventional microlens. We used three-dimensional (3D) finite-difference time-domain (FDTD) simulation to optimize the radius of curvature of gapless microlens and over-coat layer thickness. We also designed light guiding structures to reduce spatial crosstalk and increase optical efficiency. By using total internal reflection and isolated pixels, we could guide the light into over-coat layers and color filters. Using the theory of frequency selective surface (FSS), we designed UV pass filters for optical black pixels. The UV pass filter makes it possible to perform photo-diode curing process, so it can reduce dark current noise. Our works are expected to make a significant contribution to the developments of sub-micron CIS pixels.